Photocatalytic activity of amorphous-anatase mixture of titanium(IV) oxide particles suspended in aqueous solutions

Titanium(IV) oxide (TiO2) powders of various amorphous-anatase compositions were prepared by heat treatment (573-1073 K) of amorphous TiO2 in air and characterized by differential scanning calorimetry (DSC), powder X-ray diffraction (XRD), and BET surface area measurements. An exothermic peak at ca. 723 K in the DSC pattern was assigned to the crystallization of amorphous phase to anatase, and its heat was used to evaluate the weight fraction of amorphous phase. The fraction of anatase crystallites (f(anatase)) was calculated as the remainder after the amorphous phase and contaminated water or organic residue. The XRD data showed that the size of anatase crystallites was slightly decreased but almost constant along with the increase in f(anatase), being consistent with the small change in the BET surface area. These results suggest that each amorphous particle transforms into an anatase crystallite of similar size without sintering or crystal growth. The particles of mixture of amorphous and anatase were used for three kinds of photocatalytic reactions in deaerated aqueous suspensions: silver metal deposition with oxidation of 2-propanol by bare powders, and dehydrogenation of 2-propanol by ex situ and in situ platinized powders. For all the reactions, the photocatalytic activity of amorphous TiO2 was negligible, increased almost linearly with f(anatase), and further improved by calcination of completely crystallized powder. The negligible activity of amorphous TiO2 was attributed to recombination of photoexcited electron and positive hole at defects located on the surface and in the bulk of particles.